gh-36904: `src/sage/game_theory/`: fix doctest warnings due to modularization
Use more block-scope tags to avoid doctest warnings like
```
File "src/sage/game_theory/normal_form_game.py", line 1093, in
sage.game_theory.normal_form_game.NormalFormGame._gambit_
Warning: Consider using a block-scoped tag by inserting the line 'sage:
# optional - gambit' just before this line to avoid repeating the tag 29
times
threegame = NormalFormGame() # optional - gambit
```
### :memo: Checklist
<!-- Put an `x` in all the boxes that apply. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
<!-- Feel free to remove irrelevant items. -->
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
### :hourglass: Dependencies
<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
URL: https://github.com/sagemath/sage/pull/36904
Reported by: David Coudert
Reviewer(s): Matthias Köppe
